Yea, good luck finding a 2.05 ratio diff. To achieve such low cruising revs, you will need an overdrive box, and the cost and frequency of replacement clutches will far outweigh any perceived fuel economy benefits without one.

if your aiming for 2700 rpm then you can use 3.23 gears with standard height tyres, 235/65.
if the motor will have a bit of grunt behind it then you can use 2.9 that should drop the rmp down to about 2500, but the overdrive would make alot of difference too

Thanx guys - i'm going to build a decent gas engine (380-400hp) for it concentrating on max mid-range (3k-5.5k) torque (i wanted a 460, but thats a bit of overkill just to tow stuff around :) and considering u get a 9" in the donor, why not? At what hp should one think of going 9"?

The 3.25 or 3.00 sound like the most suitable - but which ratio is easiest to get as an LSD out of a wreck (ie whats most common)?

Mik

PS> Why would having a high overdrive kill a decent engine? If u get to a hill, then change down and get the revs to where it can pull... I find v8s love to just sit and tick along at (relatively) low rpm until they are needed. my 2c worth.

A) Which models (xa/xb/xc/xd/eseries and/or fairlanes) had this driveline and
b) is the length of these diffs ok to fit under the ute or are mods needed?

I would have thought a ZG-H Fairlane, or XB-C would be your best bet here for a 351 toploader and disk brake 9" (plus they're often rusty so you can find cheap "donors"). The earlier Falcs back to XW had drum brakes. Ok, not too many Fairlanes would have the 4sp, but it'd probably be harder/cost more to turn up a disk braked 9".

I don't think the leaf spring suspension changed on Falcons so a 9" out of any leaf-sprung Falc should be ok.

As for the ratio, first find the bits. Chances are it will need rebuilding anyway unless you know its history, so it's easy to change ratio then if necessary but you're most likely to find a 3.0 or around that.

Racer - not having a go, but what's wrong with having a sensible ratio diff for highway driving behind a strong 351? He's obviously not after the best possible 1/4 time. A 9" will take more punishment that the BW diff, so apart from the weight difference and a bit more cost, why not?